Why mirroring and not rotation?
In the particular case of the teddy bear in the tutorial referenced by the original poster, because the leg and bottom of the leg objects are approximately symmetrical around the center axis of the leg, mirroring the object, and rotating the object will have similar results, except that it appears that mirroring may result in the normals of the faces of the object being flipped. With a more complex mesh, however, mirroring and rotating the objects will not have the same result. An example of this would be an exterior rear view mirror on an automobile, such as this one:

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In this case, duplicating the object(s) and using the mirror tool (CTRL - M) will yield a right side rear view mirror. However duplicating the object(s) and then rotating it 180 degrees will not have the same result; instead the right side mirror will have the glass facing forward, and the mounting surface in the wrong orientation. 

Note 1: Original photo at https://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Wing_mirror_with_indicator_light.JPG/297px-Wing_mirror_with_indicator_light.JPG